Injection molding (British English: moulding) is a manufacturing process for producing parts from both thermoplastic and thermosetting plastic or other materials, including metals, glasses, elastomers and confections. Material is fed into a heated barrel, mixed, and forced into a mold cavity where it cools and hardens to the configuration of the cavity. After a product is designed, usually by an industrial designer or an engineer, molds are made by a mold maker (or toolmaker) from metal, usually either steel or aluminum, and precision-machined to form the features of the desired part. Injection molding is widely used for manufacturing a variety of parts, from the smallest component to entire body panels of cars.
Injection molding utilizes a ram or screw-type plunger to force molten plastic material into a mold cavity; this produces a solid or open-ended shape that has conformed to the contour of the mold. It is most commonly used to process both thermoplastic and thermosetting polymers, with the former being considerably more prolific in terms of annual material volumes processed.
Thermoplastics are prevalent due to characteristics which make them highly suitable for injection molding, such as the ease with which they may be recycled, their versatility allowing them to be used in a wide variety of applications, and their ability to soften and flow upon heating.
Injection molding consists of high-pressure injection of the molten plastics material, referred to as plastic melt, into a mold, which shapes or forms the polymer into a desired shape. Molds can be of a single cavity or multiple cavities. In multiple cavity molds, each cavity can be identical and form the same parts or can differ and produce multiple different geometries during a single cycle. Molds are generally made from tool steels, but stainless steels and aluminum molds are suitable for certain applications.
Aluminum molds typically are ill-suited for high-volume production or parts with narrow dimensional tolerances, as they have inferior mechanical properties and are more prone to wear, damage, and deformation during injection and clamping cycles; however, they are more cost-effective in low volume applications as mold fabrication costs and time are considerably reduced. Many steel molds are designed to process well over a million parts during their lifetime and can cost hundreds of thousands of dollars to fabricate.
When thermoplastics are molded, typically pelletized raw material is fed through a hopper into a heated barrel with a feed screw. Upon entrance to the barrel, the thermal energy increases and the Van der Waals forces that resist the relative flow of individual chains are weakened as a result of increased space between molecules at higher thermal energy states. This reduces its viscosity, which enables the polymer to flow under the influence of the driving force of the injection unit. The feed screw, typically an Archimedean screw, delivers the raw material forward, mixes and homogenizes the thermal and viscous distributions of the polymer, and reduces the required heating time by mechanical shearing of the material and adding a significant amount of frictional heating to the polymer. The material is fed forward through a check valve and collects at the front of the screw into a volume known as a shot. The shot is the volume of material, which is used to fill the mold cavity, compensate for shrinkage, and provide a cushion (approximately 10% of the total shot volume which remains in the barrel and prevents the screw from bottoming out) to transfer pressure from the screw to the mold cavity. When enough material has gathered, the material is forced at high pressure and velocity through a gate and into the part-forming cavity by moving the screw along its axis. To prevent spikes in pressure, the process normally utilizes a transfer position corresponding to a 95-98% full cavity where the screw shifts from a constant velocity to a constant pressure control. Often injection times are well under one second and the cooling time of the part in excess of four seconds. Once the screw reaches the transfer position the packing pressure is applied, which completes mold filling and compensates for thermal shrinkage, which is quite high for thermoplastics relative to many other materials. In thermal gating the packing pressure is applied until the material located in the mold gate (cavity entrance) solidifies. The gate is normally the first place to solidify through its entire thickness due to its small size. Once the gate solidifies, no more material can enter the cavity; accordingly, the screw returns and acquires material for the next cycle while the material within the mold cools so that it can be ejected and be dimensionally stable. This cooling duration is dramatically reduced using cooling lines to circulate water or oil from a thermolator or preferably using an organic refrigerant. Once the required temperature has been achieved, the mold opens and an array of pins, ejectors, etc. is driven forward to remove the article from the mold, referred to a “de-molding”. Then, the mold closes and the process is repeated. The thermal gating, where the closing of the gate is accomplished by solidified plastic, is possible for small flow requirements with a melt flow channel with small gates. For high plastic flow rates, a valve gated hot runner is used in which mechanical valves control the flow of melt from a common supply, or hot runner, to the mold. A modulating assembly modulates the melt flow. Faster cycle time may be attained because no gate cooling is required to shut off the melt flow, and no gate re-heating is required to open the gate to the melt flow.
A parting line, sprue, gate marks, valve pin marks, and ejector pin marks are usually present on the final part, often even after prolonged cooling time. None of these features are typically desired, but are unavoidable due to the nature of the process. Gate marks occur at the gate that joins the melt-delivery channels (sprue and runner) to the part-forming cavity. Parting line and ejector pin marks result from minute misalignments. The wear, gaseous vents, clearances for adjacent parts in relative motion, and/or dimensional differences of the mating surfaces contacting the injected polymer also create marks on the molded surface of the part. The dimensional differences can be attributed to non-uniform, pressure-induced deformation during injection, machining tolerances, and non-uniform thermal expansion and contraction of mold components, which experience rapid cycling during the injection, packing, cooling, and ejection phases of the process. Mold components are often designed with materials of various coefficients of thermal expansion. These factors cannot be simultaneously accounted for, without astronomical increases in the cost of design, fabrication, processing, and the part quality monitoring. The skillful mold and part designers, will position these aesthetic detriments in hidden areas, if feasible.
Inevitably, to eliminate the gate marks, it is necessary to improve gate performance. The gate marks and gate residuals are called vestige. The molding quality is directly apportioned to shape, configuration, degradation of the vestige and vestige height and shape. The gate quality is nowadays-major issue in injection molding art, particularly for food and beverage packaging.
It is well-known in the field of injection molding art that some structure must be placed in the mold gate, at a particular time in the molding cycle, to inhibit the flow of molten material into the cavity of a mold, so that the molded part may be cooled, and subsequently opened to remove the molded parts. This must be done without creating drool of the molten material in the molding surface. This drool would create undesirable marks on the next moldings, and this is largely un-acceptable.
As noted above, there are essentially two broad categories of melt flow modulating assemblies, or flow inhibiting techniques known in the field of injection molds, namely, thermal gating in which the gate at the exit of the nozzle is rapidly cooled at the completion of the injection operation to form a solid or semi-solid plug of the material being injected into the gate; and valve gating in which a mechanical means is employed to inhibit the flow of material being injected into the mold cavity.
Each category has its own advantages and disadvantages relative to the other. Numerous systems using thermal gating are known in the art of the hot-runners.
Valve gating systems are generally of one of two types, namely inline and lateral systems of gate closing. A wide variety of systems of each type have been developed. Referring now to the inline gating choices, there is in the art of the injection molding, mainly three types of valve gate closing choices: axial pin motion, rotary pin motion with shutoff, and a rotary pin with dynamic melt flow control without positive shutoff.
Many valve mechanisms used in the injection molding industry are constructed in such a way as to move a valve pin assembly in an axial direction along the nozzle melt channel from fully open to fully closed position. This is the predominant structure when comparing based on the motion of the valve pin.
An example of this is found in U.S. Pat. Nos. 4,268,240, 6,086,357, U.S. Patent publication No. 2011/0293761 A1, U.S. Pat. Nos. 8,047,836 B2, 7,600,995 B2, or for example, U.S. Pat. No. 7,044,728 B2 and U.S. Patent publication No. 2005/0100625 A1 where the plastic is transferred from a hot-runner manifold to a nozzle. This type of the melt delivery goes around the pin and then rejoins the melts from each side of the pin, and reconstitutes the tubular flow just below the valve pin tip. Therefore, this kind of the valve pin motion, being axial, causes melt flow, arriving laterally at the pin, to be divided by the valve pin or stem.
The flow is rejoined again into a single path as it passes in to the mold cavity, resulting in moldings with undesirable weld lines created by the once divided polymer volumes, visibly affecting quality of the products. These weld lines can adversely affect both the aesthetic and performance qualities of the final molded product, and it is significantly advantageous to avoid their creation when molding certain products.
Some alternatives to prevent melt separation have been proposed, e.g. the valve pin may be shielded, as in U.S. Pat. No. 4,412,807 which shows an apparatus in which the plastic flow channel in the nozzle is kept separate from the valve pin in an effort to avoid dividing the melt stream. The channel is a crescent shaped cross section, which is known to be less than ideal for encouraging plastic flow, especially in the opposing sharp corners. Furthermore, when the valve pin is in the open position to let plastic material to pass into the mold cavity, it creates a stagnant area of poor plastic flow directly adjacent the front face of the pin. These areas of poor plastic flow can result in material degradation, which can adversely affect the performance and physical properties of the molded product.
U.S. Pat. No. 4,925,384 shows a similar design that permits the plastic to come into contact with the valve pin but restricts it from passing around the pin to form a weld line. This patent describes an approach that does not cause pronounced division of the melt flow. This design also suffers from a melt channel with sluggish flow areas and requires difficult and expensive machining processes to produce the nozzle housing, having an unusual melt channel cross section.
Alternatively, valve gates may be structured to rotate the pin and close or open the gate that way.
U.S. Pat. No. 3,873,656 shows a valve having taps, which rotate to open or close. This is similar to the approach described above. It is not compact or easy to manufacture and has sharp edges, susceptible to damage, where it mates with the sprue channels.
A rotating nozzle is shown in U.K. Patent No. 872,101. The entire injection unit nozzle rotates on an axis parallel to the flow of plastic as opposed to the perpendicular or angular rotation axis of the two patents mentioned previously. The nozzle front portion remains in forced contact with the delivery bushing, to prevent plastic leakage between the two. The construction shown is very bulky, consuming a substantial amount of space.
Further example of the attempt to reduce weld line and part marks is disclosed in the U.S. Pat. No. 5,499,916 where the stem rotates with limited contact with the melt flow but does not allow melt separation.
A further example to improve melt flow delivery, and melt flow temperature uniformity as well as hot runner balancing is attempted in the application of the rotating pin is disclosed in U.S. Patent Publication No. 2007/0065538 A1. The valve pin is operatively connected to a motor that has fast acceleration and deceleration rates. The valve pin is made in the form of an Archimedean screw or screw pump so that the pin is positioned within the melt flow assembly in the hot-runner nozzle. By rotating and pumping melt flow in the direction of the melt flow, the valve-pin “pump” reduces pressure drop within the melt flow assembly, supposable creating favorable melt delivery and melt conditioning.
However, when rotation is in the direction to retard melt flow of the molten material traveling in the direction of the cavity, higher-pressure drop is created in the melt flow assembly of the hot-runner and therefore, balancing the pressure and flow to ensure that drop-to-drop uniformity is maintained. Besides the positive effect on the uniformity of the melt, that is critical for food packaging and medical parts, this valve gate molding system can effectively produce acceptable quality gate vestige mark and at the same time ensure that the closing of the gate is accomplished by rotation of the pin screw “pump” within melt in the melt flow assembly and at the same time improve temperature uniformity of the melt in the hot-runner nozzle.
In each of the systems described above, and in inline systems, generally, a valve pin aligned with the gate is moved parallel to the direction of movement of molten material (generally referred to as “melt”) through the gate, between a position wherein the pin extends into the gate to block flow through the gate, and a position wherein the pin is retracted from the gate permitting flow there-through into the mold cavity. In order to be aligned with the gate, the valve pin is located inside the injection nozzle and is at least partially within the flow path of the melt.
For these and other reasons, inline valve gating suffers from a variety of problems.
One common problem is wear of the valve pin due to contact with the nozzle and/or gate, which can lead to leaks or failure of the valve.
Another common problem is the conversion of the melt from the tubular flow entering the nozzle to an annular (or other non-continuous) flow, which is caused by the valve pin or other related components being within the melt flow. Such a non-continuous flow can result in weld or knot lines in the molded product produced as the melt flow recombines within the gate or mold cavity, and this can result in weakened or unacceptable molded products. This is particularly a problem when molding preforms for water containers where good part appearance and gate quality are an essential for successful sales of bottled water or other clear liquids.
The water bottles are made in a two-stage process and, require in a first stage to produce a preform, and in a second-stage the preform is air inflated against a cavity of the mold in a shape of the bottle. The bottle preforms are made from the polyethylene terephthalate, abbreviated as PET. The PET preform molding process, in particular, requires tubular melt flow, and having the pin inside the melt flow, does not help improve melt flow in the molding process.
During the injection process, the molten plastic material is injected into the mold cavity under very high pressure, often above 15,000 PSI. Once injected, in a short injection time, often less than one second, molten plastic enters cooling and solidification phase lasting 2 to 30 seconds. During this process, a definite time is selected, within hold time in the process, when to close the valve gate. Closing valve gate means that gate volume should be filled by pin tip volume so as to block plastic flow through the gate.
The axial movement of the valve pin assembly accomplishes this.
In principle, the valve pin conical surface and gate conical surface each have a complementary sealing surface. When these surfaces are brought together the flow of the material through the gate stops.
Usually, as it is well recognized in the injection molding art, the gate closing is initiated just about when the cavity is filled, and the injection time hold interval is about to end. After the valve pin is fully forward and in a predominantly closed position, no more plastic melt is possible to enter the cavity. Mold cooling helps to remove heat from the molded part and helps to solidify the part and cool it so that can be handled in post molding cooling process. The post molding process, by itself is the complex process when molding PET preforms or any food packaging containers like K-cups. The post molding process often requires specialty equipment and additional complexities.
The valve pin stays closed until the mold is fully open and perhaps even just before mold fully closed position after ejection of the molded part. Of course, timing when to start opening the valve gate is largely dependent on the valve pin driving apparatus.
Fast acting valve gate systems allow for more flexibility and better timing and control of the gate mechanism. Currently air piston operated valve gates require closing time up to one second due to lack of proportionality between air pressure and axial force. Once air piston start moving it only stops at the hard stop at the end of the stroke. Similarly, servo motor driven pins, introduce nonlinearity largely because gear box and nonlinearities in magnetic structures of the current servo motors and drives.
As noted above, there are various options for the valve gate pin configuration and ways for opening and closing the gate. There are, however, only a few options for powering the valve pins. It is known in the art of injection molding and hot runners to provide an electrical or fluid actuator to power the pin of the valve gate.
The electric motors, air motors or hydraulic motors mostly power the rotary pins. For axially moving valve pin, typically, the actuators are the pneumatic or hydraulic type. The moving air piston type actuator is predominantly used today to power axially moving valve gate pin due to its simplicity and compactness. All other motors, including servo motors and drives require conversion of rotary to linear motion via transmission elements or gearbox.
The disadvantage of using an air piston cylinder to power the valve gate assembly, besides extensive drilling of the substantial number of air channels, is that the pneumatic piston actuator may require specialized valves and air hoses to deliver and control the compressed air. The pressure of the air supply in each location is different and is very difficult to ensure consistent high air pressure at each valve pin location. Even when air pressure is available, often in range 75 PSI (pounds per square inch) to 120 PSI, flow rate, cleanliness and capacity of the air compressors may not be always adequate. Often just differences in hose length will change the mold performance due to different air supply pressure seen by each valve gate. Just the fact that the piston seal stiction alone, in the multi cavity mold, may be different at different operating temperatures is material and illustrates the level of randomness involved in these systems. The mechanical tolerance, location in the mold, air supply line arrangements, and environmental contamination, maybe enough to result in less than an optimal valve gate opening or closing time. These and other variations result in differences in part quality and quality of the gate vestige. These and other variations are not desirable.
Yet another disadvantage of the air operated valve gates is that the pin can only be positioned at the fully open position or at the fully closed position, and cannot be positioned between these two positions, unless additional pistons or complexities are installed. Moreover, as the compressed air temperature varies during the day, this inhibits molding good parts without continuous process adjustments and monitoring. A further disadvantage of the air piston operated valve gates is that they are relatively easy to get contaminated by the PET dust or air impurities and then get slow to move and not very accurate in the closing position of the pin.
A further disadvantage is that the air exhaust contaminates freshly molded parts, and parts for medical and food packaging industry are very sensitive to parts cleanliness.
The most important disadvantage of the air operated valve gate systems is that air is exhausted to the environment and large volume of air is used for these operations. Compressing and delivering air to the molding system is very expensive and compressed air is delivered with overall compressor efficiency less than 40%. That means only a portion of the electrical energy used for compressing and delivering compressed air is used and converted into a useful motion of the valve pin.
Hydraulic pistons are often used for large valve gated assemblies and relatively high axial force requirements, but using hydraulic oil and mist in the vicinity of freshly molded medical or food packaging parts, is not acceptable.
Electric motors with rotary motion are being used for generating axial motion.
The motors and gear transmission assemblies are very large in volume and mostly not suitable for applications with a higher number of cavities.
In some applications, like food packaging and medical molding industry, the use of the electric actuators for the valve gates is demanded due to their cleanliness. Air and hydraulics just generate too much of the air contaminant dispersion to be acceptable in clean environments like medical moldings and food packaging.
Electrical actuators are becoming more compact and being now available in a variety of the configurations, which allows them to be used as actuators for the valve gate assemblies in injection molding systems.
One example of such an electrically operated valve gate pin is disclosed in the U.S. Patent Publication No. 2005/0100625 A1. In that patent, a valve gate assembly for regulating a flow of molten material into a mold is operated by the electric motor. The electric motor operates via a mechanical transmission to move the valve pin, and infinitely positions the valve pin between the fully closed position and the fully open position by using a position feedback device in a closed loop servo control mode. Various electric motors are proposed for this application, but servo controls of this nature are largely impractical for high cavitation molds where 96 to 144 individual PET preforms may be molded within each machine cycle. Besides, a feedback device installed for each individual pin position feedback is impractical and very difficult to integrate in the molds and hot runner assemblies. Even if, and when used, the closed-loop servo motor powered valve pin, must maintain the valve pin in a closed position when the operator's gate is open to prevent hot plastic melt spray and injury to operators entering the mold area. The servomotor must maintain positioning accuracy and stiffness throughout the injection cycle, and that means high current is required to just maintain the position. Motors must be rated for 100% duty cycle. It is easy to see how overheating of the electric motor can occur, and then additional complexities must be built into a servo system to overcome that. Molders today just are not ready to put up with maintenance and servicing requirements of hundreds of the individually controlled servo motor systems, despite the valve pin positioning accuracy and associated benefits of the accurate individual valve pin positioning.
U.S. Pat. No. 5,556,582 describes the system wherein an adjustable valve pin is operated by the servo controlled motor. The valve pin can be dynamically adjusted by a computer according to pressure data read at or near the injection gate. If multiple valves are used, each is independently controlled. A hot runner nozzle is not provided. Also, as the system is used, the repetitive actions of the valve pin cause significant wear on the tip of the valve pin. This wear, is a result of the repeated impact with the mold cavity. Basically, an adjustable valve is provided that is adjusted by the close loop servo system, while the plastic melt material is flowing through the gate into the mold cavity. The computer controls the servo motor, based on a sensor in the cavity, preferably stated as being cavity pressure closed loop servo system. This control is complex and not easy to implement in large cavitation molds.
U.S. Pat. No. 6,294,122 B1 describes the system of driving the pin axially along the nozzle melt channel in a closed-loop control by operatively connecting the pin with linearly moving mechanical transmission assembly, which converts the rotational movement of the motor assembly into linear motion. The conversion assembly is a gearbox or screw and a nut threadingly engaged with each other or alternatively driven by the rack and pinion gear assembly. Positioning is based on the proportional integral and derivative (PID) controls getting position feedback from an encoder. This approach, while sophisticated and allowing for very precise valve pin positioning, is complex and overly sophisticated for the applications and the current state of the art in the plastic industry today. Besides, having transmission elements between an electrical rotor and vale pin introduces unacceptable response delay. The motor gear assembly use is therefore limited to large molds often used in automotive applications where fast pin closing moves are not required. Besides, having bulky motor and a gearbox between the molding platens of the injection machine limits the opening stroke and type of the parts that can be molded with this arrangement. Again this is generally not practical for high cavitation counts and high production rates.
In a similar attempt to operate a valve pin with a clean electrical motor and accommodate a large number of drops, U.S. Patent Publication No. 2011/0293761 A1 describes a system where a plurality of pins is attached to an electro-magnetically driven plate so the valve pins are movable responsive to movement of the actuation plate. No proposed driving logic is offered as to how to control the largely uncontrollable force at the end of the stroke. When two magnetic assemblies of the type shown in this published application get very close together, the impact and noise generated by the plate contact is likely to damage connecting elements of the valve pin if not mitigated with additional complexities. It would also likely result in a very slow movement of the valve pin assembly because it would take substantial time to establish a magnetic field in a large magnetic storage like electromagnets, and to subsequently reverse that field. To collapse the electromagnetic field in assemblies of this size is a lengthy and involved process, even when sophisticated electronic devices are used. The magnetic structures of this size and mass do not allow for fast current switching, because the collapsing magnetic field and changing polarity will generate back electromotive force of significant proportions. Simply, a large mass does not lend itself for fast opening and closing valve pins.
At the opposite spectrum of valve pin actuations, small electromagnetic actuators have been proposed and tested. The most promising method of direct pin activation is the method of controlling pin closed and pin open position with two solenoids but aided by a spring: one to hold the valve open and one to hold the valve closed. Since the electromagnetic solenoid actuators are inherently unidirectional and a large force is required at the end of the stroke, it is very difficult to electronically control the movement of the pin.
Also the force exerted by these solenoid actuators is proportional to the square of the current input, and decreases as the function of the air gap between the actuator and the armature. Therefore, as good as these actuators are, their control is difficult for consistent operation. Having a mechanical spring is also an undesirable feature.
It is critical for the valve pin to arrive exactly at the end of the stroke with exactly near zero velocity. This is often defined in state of the art as perfect “soft landing”. The receiving end actuator must do exactly as much work as was done against friction and adhesive force of the melt along the entire transition from open-to-close or vice versa. If the actuator does not do this much work, the valve pin will stop before the end of the stroke. If the actuator does any more than the exact correct work, the valve pin arrives at the end of the stroke with non-zero velocity where it can impact valve seat if contacts it, or imparts the shock and vibration on the valve pin assembly by impacting against a hard stop. The non-uniform force, and other effects cause disturbances of the valve pin assembly and make this system very difficult to control.
None of the foregoing valve pin activation and control techniques offer individually controlled valve pin structure in a small and compact size that will move the pin axially along the melt flow channel, without any interconnecting, converting mechanical transmission elements to reduce speed, or convert power or convert torque. These and other systems require installation of the position or process feedback devices in areas that has limited space. The mechanical structures have very small structural safety margins, and any additional requirements for installation of any feedback devices make the systems very complex. This is particularly difficult in applications with an increased number of cavity drops and reduced drop to drop (pitch) spacing.
The above discussion exemplifies the challenges in controlling a manufacturing process to produce components at a high production rate with a consistently high quality. Attempts to maintain machine operations with fixed input parameters inevitably leads to a variation of quality as ambient and input conditions change. For example a small change in the characteristics of the feedstock, or a change in the ambient temperature in which the process is performed, may produce a deleterious effect on the component being produced, which will lead to rejected components if an adjustment is not made.
Attempts to compensate for such variation utilize feedback controls, either open loop or closed loop systems. Feedback systems have part of their output signal “fed back” to the input for comparison with the desired set point condition. The type of feedback signal can result either in positive feedback or negative feedback.
In a closed-loop system, a controller is used to compare the output of a system with the required condition and convert the error into a control action designed to reduce the error and bring the output of the system back to the desired response. Then closed-loop control systems use feedback to determine the actual input to the system and can have more than one feedback loop.
Closed-loop control systems have many advantages over open-loop systems. One advantage is the fact that the use of feedback makes the system response relatively insensitive to external disturbances and internal variations in system parameters such as temperature. However it has been found through experiments and customer feedback that close controls executed in a classical way do not produce the adjustments necessary to maintain good quality articles. For example, in the case of temperature, the sensor is the thermocouple. (TC). It measures the temperature at the single point and controller is attempting to maintain the same temperature at that point. Even if this is possible, quality of the final product may not be good, because of the very many variables that can affect the part physical dimensions, and perhaps flow characteristic of the melt.
It is therefore an object to the present invention to obviate or mitigate the above disadvantages.